1. Field of the Invention
The present invention relates to an injection mold and an injection molding apparatus, and more particularly to an injection mold and an injection molding apparatus for molding a small and light optical element, such as a lens.
2. Description of Related Art
In recent years, with improvement of resin materials and injection molding techniques, small and light resin lenses have been developed, and a demand for use of these resin lenses as optical elements in optical pick-up devices and portable telephones has been stronger. In order to produce such resin lenses, molds which permit accurate transfer of fine configurations for diffraction elements and smooth surfaces or the like are required.
Generally, a gate of a mold is narrow, and in a pressure retention process after injection of resin into a cavity, the resin in the gate is solidified relatively early. Therefore, the pressure applied to the resin in the cavity becomes weaker, and a supply of resin to compensate shrinkage of the resin becomes short. Consequently, a molded product has problems that the accuracy of a fine configuration of a surface and the accuracy of a smooth surface are low (low transfer accuracy) and that since a tensile stress is applied to the center part of the product due to shrinkage of the resin, the birefringence of the lens becomes larger.
Japanese Patent Laid-Open Publication No. 2002-187168 discloses a mold for molding an optical element with high surface accuracy and with little residual deformation. This mold has a sliding mechanism to slide the side portions of the mold, except for the sides of the gate, so that a molded product can be released from the mold smoothly. However, this mold does not solve the above-described problems, namely, a degradation in transfer accuracy caused by solidification of resin in the gate and an increase in tensil stress caused by shrinkage of resin in the cavity.